Light phase grating light valves are micro-mechanical devices that are useful in display applications. These devices are generally used in combination with a light source to form selective images on a display. In particular, in response to an applied voltage, these devices either reflect or diffract light emanating from the light source. When a plurality of these devices are combined into an array separated by equally-spaced slots, in a similar manner that the pixels on a display are formed in an array, these devices can be selectively controlled to form selective images on the display.
The light phase grating light values are particularly useful for digitally controlled displays. In their general configuration, an absence of an applied voltage to these devices places them in a light reflecting mode. Conversely, when a voltage is applied to these devices, these devices are placed in a light diffracting mode. Because of the dual mode function of these devices, they easily lend themselves to be digitally controlled. That is, when the applied voltage is a "low" or a logical "zero" (0), the light phase grating device is controlled to be in its reflecting mode. Conversely, when the applied voltage is a "high" or a logical "one" (1), the light phase grating device is controlled to be in its diffracting mode. Again, when a plurality of these devices, separated by equally-spaced slots, are formed into an array, digitally controlling each of these devices can be performed to form selective images on the display.
FIG. 1 is a top diagram view of a prior art light phase grating device 10 including beam regions 11 and slot regions 13.
FIGS. 2A and 2B show the light phase grating device 10 of FIG. 1 along a cross-section A--A for its reflecting and diffracting modes, respectively. The device 10 is typically formed on an insulating layer 14 over a silicon substrate 12 and includes a first conducting layer 16 that is disposed on an insulating layer 14 that is, in turn, disposed on the substrate 12. The device 10 further includes a beam 22 that consists of a top, thin-film conducting layer 22a and a bottom, thin-film insulating layer 22b. The beam 22 is suspended over the first conducting layer 16 by a pair of insulating supports 20. Furthermore, the grating valve consists of numerous such beams, equally separated from each other by slots, as shown in FIG. 1. The film layers in each slot include the substrate 12, a dielectric region 12, a conducting region 16, and a highly reflective layer, which is typically deposited at the same time as layer 22a on the top of the beams.
In order to operate the device 10 in either its light reflecting or light diffracting modes, the device is connected to a voltage power supply, such as battery 24 shown for illustrative purposes. Specifically, one of the terminals of the battery 24 is electrically coupled to the conducting layer 22a of the beam, and the other terminal is electrically coupled to the first conducting layer 16.
When no voltage is applied to the device 10, such as shown in FIG. 2A where the positive terminal of the battery 24 is not connected to the conductive layer 22a, the beam 22 remains suspended over and not in contact with the first conducting layer 16. Since in this position the beam remains substantially horizonal, the top surface of conducting layer 22a, in conjunction with the film 22a in the slot regions, forms a substantially light reflective surface. The distance between the top of the slots and the top of the beams is such that incident light strikes the top surface of beam conducting layer and the top of the slots conducting layer will constructively interfere when there is no voltage applied to the device, which results in the beam 22 being in a suspended state.
When a voltage is applied to the device 10, such as shown in FIG. 2B where the battery 24 is connected to both the first conducting layer 16 and the top beam conducting layer 22a, the beam 22 deflects in a downward direction until the insulating layer 22b comes in contact with the first conducting layer 16. In this position, the beam conducting layer 22a is pulled to the top surface of conducting film 16. The distance between the top of the slots and the top of the beams is such that incident light that strikes both the top surface of the beam conducting layer and the top surface of the slots' conducting layer destructively interferes when the beam contacts the conducting film 16.
The problem with the prior art light phase grating device 10 is that it is unreliable. Specifically, the problem with the device 10 lies in that often the beam 22 sticks to the first conducting layer 16 and does not return to its suspended state after the applied voltage is removed. This causes the device 10 to be in its diffracting mode when it should be in its reflecting mode. Sometimes this problem is intermittent, in other words, sometimes the beam 22 sticks to the first conducting layer 16 and other times it does not. At other times, it can be catastrophic: the beam 22 always sticks to the first conducting layer 16. This results in the inability to properly control the device 10, thereby resulting in an inability to properly control the images on a display as the image changes.
The sticking problem of the prior art device 10 is due to triboelectric effects. Triboelectric effects occur when an insulating surface that is in contact with a second surface is pulled away from that second surface. When this occurs, the insulating layer becomes charged. This occurs when the second surface is conducting or insulating. In the light phase grating device 10, the beam insulating layer 22b builds up charges and charge states when it is pulled off of the first conducting layer 16. When the beam insulating layer 22b is frequently pulled off of the first conducting layer 16 during the operation of the device, a large amount of charges and charge states accumulate on the beam insulating layer that are mirrored with a mirror image charge on the first conducting layer 16. This results in a sufficient electric field between the insulating layer 22b and the conducting layer 16 that causes both layers to be so attracted to each other that they stick together, even when the applied voltage is removed. This results in a failure of the grating operation of the device 10.
A prior art attempt to solve this triboelectric sticking problem is to form the first insulating layer 16 in a manner that it has a rough top surface. This reduces the effective contact area between the beam insulating layer 22b and the first conducting layer 16. As a result, fewer charges are formed on the insulating layer as the layer is pulled off of the first conducting layer 16. This solution, however, reduces the problem, but it does not eliminate it. In other words, the light phase grating device 10 will have a longer operational and more reliable life span than if the top surface of first conducting layer 16 was not rough.
Therefore, there is a need for a light phase grating device that eliminates the problem of the beam sticking to the first conducting layer 16 due to triboelectric effects.